Spider Web Is Strong and Elastic

black, yellow, and white spider on web during the daytime

Biological Strategy

Eleanor Banwell

Image: Amandad / Pixabay / Free non-commercial use

Capture, Absorb, or Filter Organisms

Many living systems must secure organisms for food. But just as one living system must capture its prey to survive, its prey must escape to survive. This results in capture and avoidance strategies that include trickery, speed, poisons, constructed traps, and more. For example, a carnivorous plant called the pitcher plant has leaves formed into a tube that collect water. Long, slippery hairs within the tube face downward. When insects enter the tube seeking nectar, they lose their footing and slide inside, unable to climb out and escape being eaten and digested by the plant.

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Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Arachnids

Class Arachnida (“spider”): Spiders, mites, ticks, scorpions

Arachnids get a bad reputation, but they play an important role in the ecosystem by controlling insect populations. Almost half of the species in the class are spiders, and most live in terrestrial environments. Unlike insects, most arachnids have two body sections: the cephalothorax, which is the head and thorax fused together, and the abdomen. Mites and ticks, on the other hand, have only one body section that is relatively flat. This body shape comes in handy for the species that are parasitic, because it makes it harder for their hosts to bite or scratch them off.

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Webs of orb spiders are elastic and strong because of sacrificial bonds that break and then reform

Spider capture silk is the silk used to form the spiral in webs built by orb weaver spiders. It is a remarkable material, as strong as Kevlar yet elastic at the same time. Orb weaver capture silk has a tensile strength of 1 GPa but can stretch to 500-1000% its original length before rebounding perfectly to regain its original properties. This is very important in spiral webs, which must be able to survive insects flying into them at speed without breaking and must also not become baggy and loose after being stretched.

Spider silk is made up of long molecules called fibroin. Parts of the molecule are disordered, however in others it folds neatly into pleated sheets called beta-sheets. The sheets are held in place by networks of very weak bonds between hydrogen and oxygen atoms. The physical shape of the protein means that it is most likely to adopt the configuration that forms the greatest number of these weak bonds, however, because they are so weak, they are easily broken.

Even paper can be made elastic by folding it into a concertina shape. In the same way, the pleated sheets of the silk protein pull apart, for instance because of impact by a flying insect. This causes the weak hydrogen bonds to break and allows the silk fiber to stretch. However, it most easily adopts the folded conformation and so once the energy of the impact has dissipated, it goes back to its original shape, allowing the hydrogen bonds to reform.